As feature sizes decrease in semiconductor manufacturing, using more than one photolithography exposure operation in one layer becomes more common. When multiple photolithography exposure operations are used, a specified overlay margin is built into the design so that the device formed does not become defective because of small alignment errors. However, such specified overlay margin limits a reduction in feature sizes as the overlay margin becomes larger in comparison to the feature size.
Smaller features also increase a use of deep implantation to implant a same amount of dopants in a smaller surface area. In one example, deep implantation is increasingly being used to form photosensitive regions and surrounding features as the surface area of photodiodes decrease. In other cases, deep implantation is used to form isolation regions in, for example, high voltage and/or high frequency applications. Deep implantation uses a higher implantation energy to drive the dopants deeper into the surface area; correspondingly, the higher implantation is applied to the implantation mask area. As result, a thicker implantation mask is used to prevent unwanted doping of areas under an implantation mask. Together with the smaller feature being doped, the aspect ratio of openings in implantation masks increases. The high aspect ratio opening has a tapered profile using conventional dry etch techniques, which reduces an effectiveness of the implantation mask and thus the maximum implantation energy that can be applied without unwanted doping. Therefore, improvements in deep implantation methods for multiple deep implantations at high energies continue to be sought.